1. Field of Invention
This invention relates to universal joints, and more particularly, to universal joints having a plurality of rollers moveable in the circumferential and axial directions.
2. Prior Art
Shown in FIG. 1 is a universal joint of the prior art. In this universal joint secured to one end of a first shaft 1 is an outer race 2 in which tripod shafts 3 disposed radially from the axis of the shaft 1 are fixed. The tripod shafts 3 are crowned with cylindrical inner surfaces 6 of rollers 5 each formed at the outer periphery thereof with a spherical surface 4.
A second shaft 7 is enlarged at one end thereof to form a large diameter portion where three cylindrical raceways 8 are formed in parallel with the axis of the shaft 7. Torque is transmitted between the first shaft 1 and the second shaft 7 by crowning the rollers 5 with the cylindrical inner peripheral surfaces 8.
When torque is transmitted on condition that the first shaft 1 and the second shaft 7 are inclined with respect to each other, rollers 5 roll about the axis of the respective tripod shafts 3 and move axially of the tripod shafts 3. The rollers 5 also move longitudinally of the cylindrical receways 8 of the second shaft 7 to transmit torque between the two shafts and therefore function as a constant velocity universal joint wherein the instantaneous centers of rotation of the shafts 1 and 7 are gradually changed so that the angular velocities of rotation of both shafts are identical with each other.
Shown in FIG. 2 is another universal joint of the prior art. In this embodiment the tripod shafts 3 are secured to the second shaft 7 and rollers 5 are supported in the cylindrical raceways 8 formed in the first shaft 1. The roller 5 functions similar to those in the embodiment shown in FIG. 1.
When a universal joint of this type is utilized on condition that the first shaft 1 and the second shaft 7 are inclined with respect to each other, the instantaneous center of rotation of both shafts do not coincide as in the case described above and the instantaneous center of rotation of both shafts are always located at positions different from each other so as to make the angular velocities of rotation of both shafts identical.
In this case, the rollers 5 makes relative sliding motion about the axis of the respective tripod shafts 3 and axially of the cylindrical raceways. Therefore, the rollers 5 wear on the coupling surfaces and seizure prematurely occurs, and the life of the universal joint is primarily determined by the wear between the roller 5 and the cylindrical raceway 8.
In order to overcome this problem, there have been adopted various counter-measures in the prior art. Such counter-measures include improvements in surface roughness of the sliding surfaces, decreases in surface pressure and improvements in lubrication. However, these counter-measures do not include direct lubrication onto the sliding surface and are ineffective.